Exam 15: Partial Derivatives

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Use Taylor's formula to find the requested approximation of f(x, y) near the origin. -Cubic approximation to f(x,y)=ex+2yf ( x , y ) = e ^ { x + 2 y }

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Find the extreme values of the function subject to the given constraint. - f(x,y)=xy,9x2+4y2=36f ( x , y ) = x y , \quad 9 x ^ { 2 } + 4 y ^ { 2 } = 36

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Solve the problem. -Find the derivative of the function f(x,y)=x2+xy+y2f ( x , y ) = x ^ { 2 } + x y + y ^ { 2 } at the point (4,5)( 4,5 ) in the direction in which the function increases most rapidly.

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Solve the problem. -Find the least squares line through the points (1, -4) and (2, 2).

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Find the extreme values of the function subject to the given constraint. - f(x,y)=xy,x2+y2=32f ( x , y ) = x y , \quad x ^ { 2 } + y ^ { 2 } = 32

(Multiple Choice)
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Estimate the error in the quadratic approximation of the given function at the origin over the given region. - f(x,y)=sin3xsin24y,x0.1,y0.1f ( x , y ) = \sin 3 x \sin ^ { 2 } 4 y , | x | \leq 0.1 , | y | \leq 0.1

(Multiple Choice)
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Solve the problem. -About how much will f(x,y,z)=xy3z2f ( x , y , z ) = x y ^ { 3 } z ^ { 2 } change if the point (x,y,z)( x , y , z ) moves from (3,7,6)( 3 , - 7 , - 6 ) a distance of ds=110\mathrm { ds } = \frac { 1 } { 10 } unit in the direction of 2i+2jk2 \mathbf { i } + 2 \mathbf { j } - \mathbf { k } ?

(Multiple Choice)
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Give an appropriate answer. - f(x,y)=x37x2y6xy3f ( x , y ) = x ^ { 3 } - 7 x ^ { 2 } y - 6 x y ^ { 3 }

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Find two paths of approach from which one can conclude that the function has no limit as (x, y) approaches (0, 0). -We say that a function f(x,y,z)f ( x , y , z ) approaches the limit LL as (x,y,z)( x , y , z ) approaches (x0,y0,z0)( x 0 , y 0 , z 0 ) and write (x,y,z)(x0,y0,z0)f(x,y,z)=L( x , y , z ) \rightarrow \left( x _ { 0 } , y _ { 0 } , z _ { 0 } \right) \quad f ( x , y , z ) = L if for every number ε>0\varepsilon > 0 , there exists a corresponding number δ>0\delta > 0 such that for all (x,y,z)( x , y , z ) in the domain of ff , 0<(xx0)2+(yy0)2+(zz0)2<δf(x,y,z)L<ε0 < \sqrt { \left( x - x _ { 0 } \right) ^ { 2 } + \left( y - y _ { 0 } \right) ^ { 2 } + \left( z - z _ { 0 } \right) ^ { 2 } } < \delta \Rightarrow | f ( x , y , z ) - L | < \varepsilon . Show that the δε\delta - \varepsilon requirement in this definition is equivalent to 0<xx0<δ,0<yy0<δ0 < \left| x - x _ { 0 } \right| < \delta , 0 < \left| y - y _ { 0 } \right| < \delta , and 0<zz0<δf(x,y,z)L<ε0 < \left| \mathrm { z } - \mathrm { z } _ { 0 } \right| < \delta \Rightarrow | \mathrm { f } ( \mathrm { x } , \mathrm { y } , \mathrm { z } ) - \mathrm { L } | < \varepsilon .

(Essay)
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At what points is the given function continuous? - f(x,y)=tan(x+y)f ( x , y ) = \tan ( x + y )

(Multiple Choice)
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Determine whether the given function satisfies a Laplace equation. - f(x,y,z)=5x23y22z2f ( x , y , z ) = 5 x ^ { 2 } - 3 y ^ { 2 } - 2 z ^ { 2 }

(True/False)
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At what points is the given function continuous? - f(x,y,z)=yzcos(1x)f ( x , y , z ) = y z \cos \left( \frac { 1 } { x } \right)

(Multiple Choice)
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Use Taylor's formula to find the requested approximation of f(x, y) near the origin. -Quadratic approximation to f(x,y)=11+6x+yf ( x , y ) = \frac { 1 } { 1 + 6 x + y }

(Multiple Choice)
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Solve the problem. -Find the derivative of the function f(x, y, z) = ln(xy + yz + zx) at the point (-2, -4, -6) in the direction in which the function decreases most rapidly.

(Multiple Choice)
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Solve the problem. -Find an equation for the level surface of the function f(x,y,z)=ln(xyz)f ( x , y , z ) = \ln \left( \frac { x y } { z } \right) that passes through the point (e4,e12,e3)\left( \mathrm { e } ^ { 4 } , \mathrm { e } ^ { 12 } , \mathrm { e } ^ { 3 } \right)

(Multiple Choice)
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Find the limit. - lim(x,y)(0,0)10x2+4y2+110x24y2+4\lim _ { ( x , y ) \rightarrow ( 0,0 ) } \frac { 10 x ^ { 2 } + 4 y ^ { 2 } + 1 } { 10 x ^ { 2 } - 4 y ^ { 2 } + 4 }

(Multiple Choice)
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Solve the problem. -Find the equation for the tangent plane to the surface -7x - 6y - 5z = -11 at the point (1, -1, 2).

(Multiple Choice)
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Solve the problem. -Find the point on the line 2x + 3y = 5 that is closest to the point (1, 2).

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Solve the problem. -Evaluate dwdt\frac { \mathrm { dw } } { \mathrm { dt } } at t=5\mathrm { t } = 5 for the function w=eylnx;x=t2,y=lnt\mathrm { w } = \mathrm { e } ^ { \mathrm { y } } - \ln \mathrm { x } ; \mathrm { x } = \mathrm { t } ^ { 2 } , \mathrm { y } = \ln \mathrm { t } .

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 Find fx,fy, and fZ\text { Find } \mathrm { f } _ { \mathbf { x } } , \mathrm { f } _ { \mathbf { y } } \text {, and } \mathrm { f } _ { \mathbf { Z } } - f(x,y,z)=cosyxz2f ( x , y , z ) = \frac { \cos y } { x z ^ { 2 } }

(Multiple Choice)
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